1. Field of the Invention
The present invention pertains to catalysis of chemical reactions such as the oxidation of organic materials. More specifically, it relates to the use of ash by-products from the combustion of coal, and/or compositions substantially identical to such by-products, as oxidative catalytic surface layers for the liners of cooking devices. The invention also specifically relates to substantially non-porous catalytic layers suitable for use in such cooking devices.
2. Description of the Prior Art
The so-called "self cleaning" ovens require temperatures in excess of 750.degree. F and utilize the mechanism of pyrolysis to effect removal of food spatters from the surface of oven walls. "Continuous cleaning" ovens incorporate oxidizing catalysts on the surface of the oven walls to effect such removal at temperatures in the normal cooking range of 350.degree. to 550.degree. F.
It is generally proposed that the continuous cleaning oven surfaces absorb oxygen from the atmosphere, which oxygen is either stored or diffused throughout the catalytic coatings thereon. The excess oxygen within the surface layer is believed to supply food spatters with an atmosphere of essentially pure oxygen, effectively reducing the temperature required for the oxidation of food soils in contact with the catalytic surface.
Stiles U.S. Pat. No. 3,266,477 describes a process in which catalytic oxide powders (including oxides of cobalt, nickel, cerium, ruthenium, palladium and platinum) are either dusted on a pre-fired enamel-coated metallic plate and then sintered to the glass surface or are applied by dipping enameled specimens in metal chloride solutions and drying at 400.degree. C. The reference describes the importance of retaining the catalytic particles on the surface and not allowing them to become deeply embedded in the bonding media because access to embedded particles would be restricted by the bonding media, rendering them relatively catalytically inactive.
The more recent Lee U.S. Pat. No. 3,598,650 describes a process that assertedly provides improved catalytic activity through incorporating, by smelting, high concentrations of certain oxidation-inducing metal oxides into the glass matrix of a frit and subsequently applying the material as a porcelain enamel to oven liners or walls. Because the catalytic material is homogeneously dispersed through the thickness of the coating in such a process, normal abrasion or wear on the surface ordinarily will not remove the catalyst as is the case with use of a catalytic surface layer of the type described by Stiles.
The types of catalytic coatings described by Lee and by Moreland, U.S. Pat. No. 3,587,556, that is, ones having a catalyst dispersed homogeneously through a ceramic coating, are in common use in industry today. Originally these coatings were applied to fired, ground-coated plates and then refired--a two-coat, two-fire process. Presently, however, there is a trend toward a one-coat, one-fire process in which the catalytic material (with some additions to improve adherence) is applied directly onto sheet steel base layer plates and fired.
Catalytic enamal frits are ordinarily ground in a ball mill (with mill additions to improve the rheological properties of the slip) to a fineness of about 0.2% retained on a 325 mesh screen and applied to ground-coated plates by spraying or flow coating.
For one-coat "direct-on" use, aluminum metal is usually added to the catalytic enamel slip and blended in a low shear blunger. The aluminum metal in the catalytic coating improves the adherence by limiting the crack propagation at the iron-enamel interface and by improving the mechanical properties of the enamel.
Compositions of catalytic materials and especially ceramic coatings vary widely, but the base enamel is usually an alkali-boro-silicate glass with 5 to 70% of a metal oxide catalyst and small additions of various opacifiers and nucleating agents to promote crystallization. The metallic oxides are usually combinations of the costly first period transition metal oxides.
The porosity of the coating is believed to affect both the mechanical properties and the cleaning performance of the coating. While very highly porous coatings are generally believed to increase the surface area available for oxidation to take place, and hence provide optimal oxidation characteristics, they ordinarily have poor mechanical properties and are easily removed during moderately abrasive cleaning processes.